Lerpn

- Remove unit and clipboard support
I rarely use these, so they don't get tested!
- Factorial, permutations, combinations
- Redo
- Single-key commands for logarithms
- Hexadecimal, octal, binary
- More stack functions
- 'repr, 'str commands for debugging
- Clean up prompt code, possibly removing bugs

- Modulo
- Natural and fixed-point modes
- Added rotate up, rotate down, yank from index, move to index
- Added nested stacks
- Added complex numbers